The invention relates to a motor assembly and a hoist drive including the motor assembly in question.
In certain operating situations, such as blackouts, an emergency load lowering needs to be carried out by a hoist drive. There are several known ways to perform an emergency load lowering. An emergency load lowering may be performed by controlling the brake in the hoist drive in such a manner that the lowering rate of a load is maintained within the allowed limits during the emergency lowering. Publication EP 0 588 234 discloses one of the known hoist drives in which a three-phase asynchronous machine of the hoist drive is used during an emergency lowering as a self-excited generator, and the power produced by generator operation during the emergency lowering is consumed by a load resistor assembly consisting of identical load resistors connected in a triangle.
The manual performing of an emergency lowering by adjusting the brake of the hoist drive is problematic because it requires controlling the brake manually from a service platform. Manual control is demanding, and if the emergency lowering should fail, the operator may find himself in a serious danger situation. The problem in the hoist drive of publication EP 0 588 234 is that an emergency lowering can only be optimized for a load of a particular size. If the load is substantially smaller at emergency lowering than the load for which the hoist drive is optimized, the voltage of the asynchronous machine will drop so low that self-excited generator operation ends whereby the load starts accelerating as a result of the earth's gravity. Once the speed has risen high enough, generator operation may be resumed. This, however, requires very big excitation capacitors.